1. Field of the Invention
The present invention relates to a pulse noise canceler applied to an FM tuner and, particularly, an automobile audio tuner, applied to mobile reception.
2. Description of the Related Art
FIG. 1 is a block diagram showing the arrangement of a conventional noise canceler applied to an FM tuner. An FM signal Sin is demodulated by an FM wave detector 101, and an output from the FM wave detector 101 is supplied to an FM pulse noise detector 102 and a delay circuit 103. The pulse noise sensibility of the pulse noise detector 102 is set by an attenuator 104, and a detection signal from the attenuator 104 is input to a variable control amplifier (VCA) 106 through a high-pass filter 105. In addition, an output from the VCA 106 is full-wave-rectified by a full wave detector 107. The output from the full wave detector 107 is fed back to the VCA 106 through a low-pass filter 108. This arrangement for feedback control is called a noise AGC (automatic gain controller) (109). The pulse noise detection signal is supplied to a one-shot multivibrator 110 through the noise AGC 109.
The delay circuit 103 delays a composite signal, including a pulse noise and output from the FM wave detector 101, by a time corresponding to a time required for causing the FM pulse noise detector 102 to operate the one-shot multivibrator 110. An output from the delay circuit 103 is supplied to a holding gate 111 for holding an input amplitude obtained immediately before a functional operation is performed. The holding gate 111 is opened for a predetermined period of time by a signal from the one-shot multivibrator 110 to control transmission of a signal. More specifically, during a period in which a level determined as a pulse noise by the FM pulse noise detector 102 is kept, an FM signal 10 from the delay circuit 103 is pre-held not to transmit the pulse noise to an audio signal terminal Sout.
In a received field strength at which limiter characteristics are out of THD (total harmonic distortion) including noise of an output from an FM tuner is called a threshold point Vth. This threshold point Vth is generally set to be about 10 dB.mu.V in an automobile audio tuner at present.
FIG. 2 shows the curves of noise and THD levels. The curves show the threshold point Vth of the overall tuner described above. In addition, FIGS. 3A to 3D are graphs showing the relationship between a threshold line Vth1 of an FM wave detector set depending on the threshold point Vth and an FM detection band, the relationship between the threshold line Vthl and over deviation level detection, and the relationship between the threshold line Vth1 and an FM detection output. When the FM tuner receives an electric field having a strength lower than the threshold point Vth, FM demodulation distortion is abruptly increased by an IF (intermediate frequency) filter or the like for determining the minimum bandwidth of the tuner itself. In particular, when the FM tuner receives an FM deviation wave having a frequency exceeding the bandwidth of the IF filter or the like, the FM demodulation distortion is abruptly increased. More specifically, in FIG. 2, when Vd&gt;Vth is almost satisfied, the increase rates of THD curves THD1 and THD2 are relatively proportional to the increase rate of the noise curve. However, when Vd&lt;Vth is almost satisfied, an over deviation distortion 115 whose increase rate is not proportional to the increase rate of the noise curve is formed as in THD curve THD2.
More specifically, the spectra of over deviation distortions (harmonics) range in harmonics of higher orders, and have a frequency bandwidth (FIG. 3A). In this case, when Vd&gt;Vth is almost satisfied, and the input level of the FM wave detector exceeds the threshold line Vth1, therefore a hatched portion 117 is removed by a limiter action, and no amplitude is found in an over deviation level detection output. However, when Vd&lt;Vth is almost satisfied, an over deviation output component 118 having a level and a frequency which are set in accordance with the magnitude of over deviation (FIG. 3B). However, even when Vd&gt;Vth is almost satisfied, when an over deviation level exceeding the bandwidth of the IF filter improved by the limiter action is output, the above drawback may be caused.
The spectra of the harmonics including an over deviation output 125 fall within the passband of the high-pass filter 105 in the FM pulse noise detector 102 in FIG. 1. Therefore, a harmonic is erroneously detected according to a conventional FM pulse noise detecting technique. This erroneous detection will be 10 described below. As shown in FIG. 3C, when an FM wave supposed to be over-deviated is input to the FM wave detector 101, an input to and an output from the FM wave detector 101 exhibit a transmission curve 121 when Vd&gt;Vth is almost satisfied, and a transmission curve 122 when Vd&lt;Vth is almost satisfied. An output actually obtained from the FM wave detector 101 exhibits a curve 123 when Vd&gt;Vth is almost satisfied, and a curve 124 when Vd&lt;Vth is almost satisfied (FIG. 3D). More specifically, when the over deviation output component 118 generated by the over deviation distortion 125 enters the passband of the high-pass filter 105 in FIG. 1, the over deviation output component 118 is erroneously detected by the noise AGC 109 through the full wave detector 107, thereby opening the holding gate 111 (a pre-holding operation is performed). The preheld FM detection output becomes a signal including the over deviation distortion and waveform defect distortion. In this manner, the THD curve THD2 in FIG. 2 becomes a curve 116 including the waveform defect distortion, an increase rate becomes more abrupt. As a result, the received sound becomes noisy sound.